A lambda control in combination with a catalytic converter is today the most effective exhaust-gas cleaning method for the spark-ignition engine. Very low exhaust-gas values can be obtained only together with ignition and injection systems which are presently available. The use of a three-way or selective catalytic converter is especially effective. This catalytic converter type has the characteristic of decomposing hydrocarbons, carbon monoxide and nitrous oxide up to more than 98% in the event that the engine is operated in a range of approximately 1% about the stoichiometric air/fuel ratio with lambda=1. The lambda indicates how far the actually present air/fuel mixture deviates from the value lambda=1, which corresponds to the mass ratio of 14.7 kg air to 1 kg gasoline, which is theoretically necessary for the complete combustion; that is, lambda is the quotient of supplied air mass and theoretical air requirement.
In the lambda control, the particular exhaust gas is measured and the supplied fuel quantity is immediately corrected in correspondence to the measurement signal by means of, for example, the injection system. A lambda probe is used as a measurement sensor which exhibits a voltage jump exactly at lambda=1 and so supplies a signal which indicates whether the mixture is richer or leaner than lambda=1. The operation of the lambda probe is based on the principle of a galvanic oxygen concentration cell having a solid-state electrolyte.
Lambda probes, which are configured as two-point sensors, operate in accordance with the Nernst principle as known per se based on a Nernst cell. The solid-state electrolyte comprises two boundary surfaces separated by a ceramic. The utilized ceramic material becomes conductive for oxygen ions at approximately 350° C. so that the so-called Nernst voltage is generated for a different oxygen component at both sides of the ceramic between the boundary surfaces. This electrical voltage is an index for the difference of the oxygen component at both sides of the ceramic. The residual oxygen content in the exhaust gas of an internal combustion engine is dependent to a great extent on the air/fuel ratio of the mixture supplied into the engine. For this reason, it is possible to apply the oxygen content in the exhaust gas as an index for the air/fuel mixture actually present.
The function and the measuring accuracy of the lambda probes is dependent, to a very great extent, on the temperature of the measuring element, that is, on the Nernst cell in the present case. The probe temperature would be subjected to intense fluctuations without additional measures because of the changing exhaust-gas temperatures and exhaust-gas quantities. Accordingly, in a manner known per se, the probe temperature is held as constant as possible. This controlled power is supplied to the probe with the aid of an electrical heater. A suitable measurement signal, which indicates the sensor temperature, is needed in order to determine the particular required quantity of heating power. As a rule, the electric internal resistance of the electrochemical Nernst cell is applied as a measurement signal. For this purpose, for example, a measurement current is applied to the internal resistance and the voltage which adjusts is determined with the aid of an evaluation circuit.
The measurement current is preadjusted via suitable dimensioning of the evaluation circuit in a manner known per se. In the components of the evaluation circuit, often tolerances which are present lead to fault influences in the measurement of the above-mentioned internal resistance and thereby affect the control accuracy of the heater control.